1. Field of the Invention
The present invention relates to a method and an apparatus for controlling line pressure in a continuously variable transmission.
2. Description of the Prior Art
A known continuously variable transmission has a V-belt running over a drive pulley and a driven pulley, each having a cylinder chamber with a piston therein and two conical discs, one conical disc of the driven pulley being secured to a drive shaft, one conical disc of the driven pulley being secured to a driven shaft, the other conical disc of the drive pulley being controllably movable in an axial direction of the drive shaft in response to an oil pressure in the cylinder chamber thereof, the other conical disc of the driven pulley being controllably movable in an axail direction of the driven shaft in response to an oil pressure in the cylinder chamber thereof. The oil pressure in the cylinder chamber of the drive pulley and that of the driven pulley is determined by a shaft control valve as illustrated in FIG. 1. Referring to FIG. 1, the known shift control valve denoted generally by the reference numeral 200 comprises a valve bore 201 and five ports, viz., a first drain port 201a, a first outlet port 201b, an inlet port 201c, a second outlet port 201d, and a second drain port 201e. Slidably disposed in the valve bore 201 is a spool 202 having four lands, viz., a first endmost land 202a, a first control land 202b, a second control land 202c, and a second endmost land 202d. The inlet port 201c is supplied with a pressure oil under line pressure via an oil conduit 203, the first outlet port 201b communicates with a cylinder or servo chamber of a drive pulley and the second outlet port 201d communicates with a cylinder or servo chamber of a driven pulley. The first drain port 201a and second drain port 201e communicates with an oil tank. The left end of the spool 202 is linked to a lever of a shift operating mechanism, not illustrated in FIG. 1. The control lands 202b and 202c are cooperable with the outlet ports 201b and 201d, respectively. Each of the outlet ports 201b and 201d is in the form of an annular recess having a width slightly greater than the width of the respective control lands 202b and 202c. Assuming that the control land 202b is in its central position relative to the outlet port 201b, the oil under line pressure supplied via the port 201 to an annular chamber defined between the lands 202b and 202c flows into the port 201b via a clearance formed by one edge of the land 202b, but a portion of the oil flows out of the port 201a via another clearance formed by the other edge of the control land 202b toward the drain port 201a. Thus, the pressure in the port 201b, viz., the pressure in the cylinder chamber of the drive pulley, is determined by the ratio in area between the clearances formed on the opposite sides of the control land 202b. Assuming that the land 202c is in its central position relative to the outlet port 201d, the oil under line pressure supplied to the annular chamber between the lands 202b and 202c flows into the port 201d via a clearance formed by one edge of the control land 202c, but a portion of the oil flows out of the port 201d via another clearance formed by the other edge of the control land 202c toward the drain port 201e. Thus, the pressure in the outlet port 201d is determined by the ratio in areas between the clearances on the opposite sides of the control land 202c. This pressure builds up in the cylinder chamber of the driven pulley via the oil conduit 205. When the control lands 202b and 202c are in their central position relative to the respective outlet ports 201b and 201d as illustrated in FIG. 1, the same pressure builds up in the oil conduit 204 as well as in the oil conduit 205 since the control lands 202b and 202c assume the same relationship with the respective outlet ports 201b and 201d. If the spool 202 is shifted to the left as viewed in FIG. 1, the pressure side clearance formed on that side of the control land 202b which is near the inlet port 201c increases and the drain side clearance formed on that side of the control land 202b which is near the drain port 201a decreases, resulting in an increase in pressure in the conduit 204 leading to the cylinder chamber of the drive pulley. At the same time the pressure side clearance of the control land 202c decreases and the drain side clearance of the control land 202c increases, resulting in a reduction in pressure in the conduit 205 leading to the cylinder chamber of the driven pulley. This results in a smaller reduction ratio. If the spool 202 is shifted toward the right as viewed in FIG. 1, there occur a reduction in pressure in the conduit 204 leading to the cylinder chamber of the drive pulley and at the same time an increase in pressure in the conduit 205 leading to the cylinder chamber of the driven pulley. This results in a larger reduction ratio.
The known continuously variable transmission has a problem that the V-belt slips upon rapid acceleration since the pressure in each of the cylinder chambers of the drive and driven pulleys drops rapidly. Upon rapid acceleration the spool 202 is shifted by the shift operating mechanism toward the right to a position illustrated in FIG. 2. As a result of this rapid rightward movement of the spool 202, a considerable amount of oil is discharged toward the drain port 201a via the drain clearance of the control land 202b which has increased its area rapidly. Consequently, the pressure in the cylinder chamber of the drive pulley drops rapidly and below its required minimum level shown by dotted line in FIG. 3, resulting in a slip between the V-belt and the drive pulley. The occurrence of the slip causes a decrease in tension of the V-belt, resulting in rapid reduction in an axial spreading force applied by the V-belt on the driven pulley. As a result, the axially movable conical disc of the driven pulley is allowed to move toward the V-belt. This movement of the movable conical disc of the driven pulley is quicker than the movement of the movable conical disc hydraulically in response to pressure build-up in the cylinder chamber of the driven pulley. The result thereof is that the pressure in the cylinder chamber of the driven pulley also drops rapidly and below its required minimum level shown by dotted line in FIG. 3, allowing the V-belt to slip. As a result, the engine races upon rapid acceleration owing to the slip of the V-belt.
For preventing the occurrence of the slip of the V-belt mentioned above, it is one possible measure to set sufficiently high line pressure taking into account the above mentioned drop in the oil pressure. However, this measure creates another problem that V blocks making up the V-belt are subject to excessively high compression stress. As a result, the V blocks are deformed and easy to wear, and the V-belt is stretched by an excessively large force, thus shortening the operating life of the V-belt. Still another probelm derived from setting the oil pressure high is that the loss of the oil pump is great, thus leading to a drop in efficiency of the entire continuously variable transmission.